The invention relates to a switched-mode power supply with a switch for applying a supply voltage to a primary coil of a transformer based on a drive signal fed to a pulse-width modulator. A first regulating signal that is dependent on an output voltage and a voltage signal that is dependent on a current through the primary coil is supplied to the pulse-width modulator for providing the drive signal.
Such switched-mode power supplies serve the purpose of supplying a consumer or load with a d.c. voltage, the load being connected to an output terminal of the switched-mode power supply at the secondary side. This d.c. voltage at the output side is to be kept substantially constant in a load-independent manner. For purposes of generating the output voltage, the primary coil absorbs energy via the supply voltage given a closed switch and outputs this energy via the secondary coil of the transformer and a rectifier arrangement to the load after the switch has been opened. Given switched-mode power supplies of fixed frequency, the switch is closed based on of the drive signal in periodical time intervals, whereby the periods of the individual drive input pulses vary in dependency on the output voltage and become longer with reduced output voltage or increasing load. What is referred to as "current-mode-regulation" is applied with respect to these switched-mode power supplies, which are particularly utilized in monitors and television apparatuses, for purposes of generating the drive input pulses in the pulse-width modulator. Thereby, the switch remains closed until the current through the primary coil or the current signal dependent on this current reaches the value of the regulating signal, which current increases subsequent to the switching-on.
Problems can arise with respect to a short-circuit at the output side when the transformer cannot output As energy to the load during the switch pauses. The current increases almost up to the value at which the switching-off has been carried out given the next closing of the switch. Therefore, the ON-time becomes extremely short given ideally functioning switched-mode power supplies. However, the signal propagation time determined by the signal processing in the pulse-width modulator--from reaching the switch-off value of the current signal until switching-off the switch--determines the minimal ON-time of the switch in practical operation. Therefore, the primary current further increases between the switch-off threshold is attained and the actual switching of the switch. When the transformer absorbs more energy during the ON-times of the switch than it can output to the secondary side in the switch pauses, the magnetization of the primary coil and therefore the primary current increases up to a point at which components are destroyed. Such known switched-mode power supplies provide a second operating point for purposes of preventing a destruction, whereby the switched-mode power supply is switched off by means of the current signal given exceeding of the operating point for the first time. Then, it can only be switched on again after a waiting period or after it has been disconnected from the network.